The generic term OFDM-CDMA will be used hereinafter to denote a system using time/frequency spreading.
FIG. 1 illustrates the general architecture of a conventional OFDM-CDMA digital transmitter for a downlink from a base station to a mobile terminal.
The bits a0(n), . . . , aK−1(n) from the K users are first sent to a respective channel coding module 100, . . . , 10K−1. The channel coding module comprises an encoder, which can, for example, be a convolutional-type encoder, a turbocoder or an LDPC (low-density parity-check code) type coder. The channel coding module also comprises a punching device and a bit interleaver. It supplies as output data b0(n), . . . , bK−1(n).
The binary data b0(n), . . . , bK−1(n) is then sent to a respective I/Q modulation unit 120, . . . , 12K−1, which modulates the encoded bits, for example by a four-state quadrature amplitude modulation QAM-4 and supplies as output data d0(n), . . . , dK−1(n).
The data d0(n), . . . , dK−1(n) from the various users is then processed by a spreading module 14.
Then, the spread signals are processed by a chip allocation module 16, which places them on a time/frequency grid.
The resulting signal is then transmitted to an OFDM modulator, which comprises in turn a serial-parallel conversion unit 18, which supplies as output data x0(n), . . . , xN−1(n), N being the number of subcarriers, an inverse fast Fourier transform unit 20, a parallel-serial conversion unit 22 and a cyclic prefix insertion unit 24. The symbols obtained are sent over a transmission channel.
For a description of the conventional OFDM techniques, it is worth referring to the article by W. ZHENDAO and G. B. GIANNAKIS entitled “Wireless Multicarrier Communications—Where Fourier meets Shannon”, IEPE Signal Processing Magazine, vol. 17, no. 3, pages 29 to 48, May 2000.
FIG. 2 details the spreading module 14 of the transmitter. Each symbol dK(n) of the user k (k=0, . . . , K−1) is first assigned an amplitude √PK by a module 26K. Then, in a module 28K, the rate is stepped up by a factor L and finally a digital filtering cK(z) is applied in a module 30k, the filtering coefficients being equal to the chips of the spreading sequence of the user k.
Each symbol dK(n) of the user k is spread by a sequence of L chips ck(Z). The spread signals of all the users are then added together by an adder module 32.
As described above, the chip allocation module 16 allocates the samples originating from the spreading module 14 over a time/frequency grid. It is assumed that the spreading factor L is equal to SF×ST, where SF is the spreading parameter in the frequency domain and ST is the spreading parameter in the time domain.
FIGS. 3 to 5 represent time-frequency grids respectively in the cases where ST=1 (FIG. 3), SF=1 (FIG. 4) and SF and ST are anything (FIG. 5).
FIG. 3 illustrates the case where ST=1. In this case, the characteristics of a conventional MC-CDMA system apply. F=N/SF data symbols are transmitted by code in an OFDM symbol.
FIG. 4 illustrates the case where SF=1. In this case, the characteristics of a conventional MC-DS-CDMA system apply. F=N data symbols are transmitted by code on ST OFDM symbols.
FIG. 5 illustrates the case where SF and ST are anything. In this case, F=N/SF data symbols are transmitted by code on ST OFDM symbols.
The chip allocation module 16 represented in FIG. 1 supplies as output a size vector N which can be expressed according to the equation (1), the size of this vector corresponding to the size of the fast Fourier transform:
                                          x            i                    ⁡                      [                                          qS                F                            +              p                        ]                          =                              ∑                          k              =              0                                      K              -              1                                ⁢                                                    P                k                                      ⁢                                          d                k                            ⁡                              [                q                ]                                      ⁢                                          c                k                            ⁡                              [                                                      pS                    T                                    +                  i                                ]                                                                        (        1        )            where:                i=0, . . . , ST−1 corresponds to the index of the OFDM symbol,        q=0, . . . , F−1 corresponds to the index of the sub-band,        p=0, . . . , SF−1 corresponds to the index of the subcarrier,        SF is the spreading factor in the frequency domain,        ST is the spreading factor in the time domain,        K is the total number of users,        Pk is the power associated with the kth user,        dk are the symbols associated with the kth user, and        ck is the spreading code for the kth user.        
After transposition into the time domain via the inverse fast Fourier transform unit 20, a cyclic prefix is added by the cyclic prefix insertion unit 24. It contains NG≧W−1 samples, where W is the maximum duration of the impulse response of the overall channel.
FIG. 6 represents the structure of a conventional OFDM-CDMA digital receiver corresponding to the transmitter illustrated in FIG. 1.
A rough synchronization module 60 handles, on the one hand, the detection of the start of the OFDM symbol and, on the other hand, the initial estimation of the offset ΔF between the carrier frequencies of the transmitter and the receiver and the offset ΔT between the sampling clocks of the transmitter and of the receiver.
This estimation is called rough in the sense that the estimation variance is high. This makes it possible to enter into the operating range of the receiver, but requires a so-called fine correction, to achieve the desired performance levels. Thus, the synchronization is carried out in two phases.
After a rough synchronization, the cyclic prefix is eliminated by a cyclic prefix suppression module 62 and the signal is conditioned into vectors of N samples ri(m), m=0, . . . , N−1, the index i indicating the number of the received OFDM symbol. The processing operations carried out to decode the symbols of the user k=0 are described below. The results obtained are presumed identical for the other users.
The samples ri(m) are then supplied to a serial-parallel conversion unit 64 and then to a fast Fourier transform unit 66 and a channel estimation unit 68, itself linked to a unit for estimating the propagation conditions of the channel 70.
The channel estimation unit 68 is also linked to a linear equalizer 72 which applies a coefficient gi[qSF+p] for each subcarrier. The equalizer 72 is linked to a correlation module 74, which supplies as output the estimated symbols {circumflex over (d)}0(n), . . . , {circumflex over (d)}K−1(n).
It will be assumed that the coefficients of the linear equalizer are calculated independently of the spreading codes. The equalizers can be MRC (maximum ratio combining), EGC (equal gain combining), ZF (zero forcing) or MMSE (minimum mean square error) linear equalizers, well known to those skilled in the art.
The estimated symbols obtained as output from the correlation module 74 can be expressed according to the equation (2):
                                                        d              ^                        0                    ⁡                      [            q            ]                          =                                            ∑                              p                =                0                                                              S                  F                                -                1                                      ⁢                                          ∑                                  i                  =                  0                                                                      S                    T                                    -                  1                                            ⁢                                                                    g                    i                                    ⁡                                      [                                                                  qS                        F                                            +                      p                                        ]                                                  ⁢                                                      c                    0                    *                                    ⁡                                      [                                                                  pS                        T                                            +                      i                                        ]                                                  ⁢                                                      x                    i                                    ⁡                                      [                                                                  qS                        F                                            +                      p                                        ]                                                  ⁢                                                      h                    i                                    ⁡                                      [                                                                  qS                        F                                            +                      p                                        ]                                                                                +                                    ∑                              p                =                0                                                              S                  F                                -                1                                      ⁢                                          ∑                                  i                  =                  0                                                                      S                    T                                    -                  1                                            ⁢                                                                    g                    i                                    ⁡                                      [                                                                  qS                        F                                            +                      p                                        ]                                                  ⁢                                                      c                    0                    *                                    ⁡                                      [                                                                  pS                        T                                            +                      i                                        ]                                                  ⁢                                                      w                    i                                    ⁡                                      [                                                                  qS                        F                                            +                      p                                        ]                                                                                                          (        2        )            where:                gi[qSF+p] is the coefficient applied by the equalizer to the pth subcarrier of the qth sub-band of the ith OFDM symbol,        c0 is the spreading code of the user 0,        the sign * denotes the conjugate complex,        i[qSF+p] corresponds to the symbols supplied as output by the fast Fourier transform unit 66 on the pth subcarrier of the qth sub-band of the ith OFDM symbol,        i[qSF+p] corresponds to the attenuation of the channel for the pth subcarrier of the qth sub-band of the ith OFDM symbol, and        i[qSF+p] corresponds to the Gaussian additive white noise sample of variance σ2 on the pth subcarrier of the qth sub-band of the ith OFDM symbol.        
The data is then sent to a flexible I/Q demodulation unit 76. The so-called “hard” I/Q demodulation operation consists in restoring the binary values transmitted based on the complex symbols derived from the linear detector. When a flexible-input channel decoder is used, the optimal values to be injected into the channel decoder are flexible values. The term “flexible values” is used to mean values that are not directly the hard values “0” or “1”. Thus, if a QAM-16 (4-bit) modulation is used, the flexible I/Q demodulation consists in calculating four flexible values corresponding to the four bits of the QAM-16 modulation. The optimal flexible (or metric) values to be injected into the flexible-input channel decoder correspond to the likelihood ratio logarithm (LRL).
The flexible I/Q demodulation operation is followed by the channel decoding process, performed by modules 780, . . . , 78K−1. These modules perform bit disinterleaving, unpunching and decoding operations (for example, via a Viterbi decoder in the case of a convolutional coder in transmission, by the Max log-MAP algorithm in the case of a turbocoder or by the Min-Sum algorithm in the case of LDPC coding). This restores the transmitted binary data.
In the state of the art, the unit for estimating the propagation conditions of the channel 70 is used to evaluate the instantaneous signal-to-interference+noise ratio (SINR) and/or the maximum spread of the delays and/or the maximum Doppler frequency. These three parameters measure the propagation conditions of the channel. Depending on the values of these parameters forwarded to the transmitter by return loop, the transmitter can decide to adapt the spreading parameters SF and ST in order to improve the transmission quality.
Nevertheless, the methods of calculating the maximum spread of the delays and of the Doppler frequency are not easy to implement. Moreover, the instantaneous SINR is not an optimal criterion in the selection of the spreading parameters when a channel coding/decoding process is used.
The concept of asymptotic SINR is now introduced. When the dimensions of the system become large in terms of the number and size of the spreading codes, the random matrices theory offers very powerful analysis tools with which to obtain explicit SINR values at the output of the equalizer, while taking into account the property of orthogonality of the spreading codes. This asymptotic regime is, nevertheless, obtained for routine values of the spreading factors (for example, size 32).
The expression of the asymptotic SINR for the user of rank 0 of an OFDM-CDMA system for the sub-band of rank q is given by the following formula:
                                          SINR            o                    ⁡                      [            q            ]                          =                                            S              0                        ⁡                          [              q              ]                                                          I              ⁡                              [                q                ]                                      +                          N              ⁡                              [                q                ]                                                                        (        3        )            where S0[·] corresponds to the power of the wanted signal after equalization and correlation, I[·] corresponds to the power of the multiple access noise created by the other spreading codes and N[·] corresponds to the power of the Gaussian noise filtered by the equalizer and the spreading sequence, with:
                                          S            0                    ⁡                      [            q            ]                          =                              p            0                    ⁢                                                                                    1                  L                                ⁢                                                      ∑                                          p                      =                      0                                                                                      S                        F                                            -                      1                                                        ⁢                                                            ∑                                              i                        =                        0                                                                                              S                          T                                                -                        1                                                              ⁢                                                                                            g                          i                                                ⁡                                                  [                                                                                    qS                              F                                                        +                            p                                                    ]                                                                    ⁢                                                                        h                          i                                                ⁡                                                  [                                                                                    qS                              F                                                        +                            p                                                    ]                                                                                                                                                        2                                              (        4        )            where:                p0 is the power applied to the user of rank 0,        L=SF×ST,        gi[qSF+p] is the equalization coefficient for the pth subcarrier of the qth sub-band for the ith OFDM symbol, this equalization coefficient depending on the transmission channel, and        hi[qSF+p] corresponds to the attenuation of the channel for the pth subcarrier of the qth sub-band for the ith OFDM symbol.        
                              I          ⁡                      [            q            ]                          =                  α          ⁢                                          ⁢                                    p              _                        ⁡                          (                                                                    1                    L                                    ⁢                                                            ∑                                              p                        =                        0                                                                                              S                          F                                                -                        1                                                              ⁢                                                                  ∑                                                  i                          =                          0                                                                                                      S                            T                                                    -                          1                                                                    ⁢                                                                                                                                                                                              g                                i                                                            ⁡                                                              [                                                                                                      qS                                    F                                                                    +                                  p                                                                ]                                                                                                                                          2                                                ⁢                                                                                                                                                                        h                                i                                                            ⁡                                                              [                                                                                                      qS                                    F                                                                    +                                  p                                                                ]                                                                                                                                          2                                                                                                                    -                                                                                                                        1                        L                                            ⁢                                                                        ∑                                                      p                            =                            0                                                                                                              S                              F                                                        -                            1                                                                          ⁢                                                                              ∑                                                          l                              =                              0                                                                                                                      S                                T                                                            -                              1                                                                                ⁢                                                                                                                    g                                i                                                            ⁡                                                              [                                                                                                      qS                                    F                                                                    +                                  p                                                                ]                                                                                      ⁢                                                                                          h                                i                                                            ⁡                                                              [                                                                                                      qS                                    F                                                                    +                                  p                                                                ]                                                                                                                                                                                                              2                                            )                                                          (        5        )            where:    α=K/L is the system load (K being the total number of users), and
      p    _    ⁢          =            1              K        -        1              ⁢                  ∑                  k          =          1                          K          -          1                    ⁢              p        k            is the average power of the interfering codes, and
                              N          ⁡                      [            q            ]                          =                              σ            2                    ⁢                      1            L                    ⁢                                    ∑                              p                =                0                                                              S                  F                                -                1                                      ⁢                                          ∑                                  i                  =                  0                                                                      S                    T                                    -                  1                                            ⁢                                                                                                            g                      i                                        ⁡                                          [                                                                        qS                          F                                                +                        p                                            ]                                                                                        2                                                                        (        6        )            where σ2 is the variance of the noise.
If the system under consideration is a multiple-cell system, the variance of the noise σ2 contains the power of the thermal noise N0 and the power received by the user of rank 0 originating from the other interfering base stations.
These different values are obtained independently of the value of the spreading codes, while taking into account the orthogonality property of the codes.
It is shown that the asymptotic SINR can be used to estimate precisely and easily the bit error rate (BER) before channel coding.
However, a real system employs a channel coder and its effect on system performance levels should be taken into account.
The article by N. MAEDA, Y. KISHIYAMA, H. ATARASHI and M. SAWAHASHI entitled “Variable Spreading Factor OFCDM with two dimensional spreading that prioritizes time domain spreading for forward link broadband wireless access”, VTC Spring 2003, pages 127 to 132, Jeju Island, Korea, April 2003, reveals the impact of the modulation and coding scheme on the performance levels (BER) of an OFDM-CDMA system, according to the parameters of the propagation channel (bandwidth and coherence time of the channel) and the spreading parameters SF and ST.
Furthermore, in the article by N. MAEDA, Y. KISHIYAMA, K. HIGUCHI, H. ATARASHI and M. SAWAHASHI entitled “Experimental Evaluation of Throughput Performance in Broadband Packet Wireless Access Based on VSF-OFCDM and VSD-CDMA”, IEEE PIMRC 2003, pages 6 to 11, the use of the instantaneous SINR measured by the above formula (3) to select the spreading parameters SF and ST is not very advantageous, because this SINR does not depend on the spreading parameters, since it is obtained by measuring the power of the wanted signal and the power of the interferences and of the thermal noise.
It cannot therefore be used to directly select the optimal spreading parameters SF and ST when the system uses a channel coding/decoding module. In practice, there are several SINR values representing one and the same coded data block incoming to the channel decoder. These multiple SINR values are due to the fact that the propagation channel varies in time and frequency within a coded information block. It is consequently ineffective to optimize the SINR given by the formula (3) because the channel coding and modulation must be taken into account.